Polycrystalline Si Oxidation

EE243 Sp2011 Lec 5

Polycrystalline Si Oxidation

poly-Si

grain boundaries (have lots of defects).

SiO2

roughness

with Xox

fast slower

a

b

Overall growth rate

is higher than

single-crystal Si

SiO

2

Nathan Cheung 1

Nathan Cheung EE243 Sp2011 Lec 5 2

Solution: (i) Grow sacrificial gate oxide to consume the nitride spots,

(ii) Strip sacrificial oxide ,

(iii) Regrow good gate oxide

KOOI Effect

“White

Ribbon”

Si3N4 + 6 H20 3SiO2 + 4 NH3

3 Si + 4 NH3 Si3N4 + 6 H2

Nathan Cheung 3

Bird’s Beak and Kooi Effect

Kooi (White Ribbon)

effect that requires the

gate oxide to be grown

twice. (N related; etch

hot H3PO4)

LOCOS Bird’s Beak

EE243 Sp2011 Lec 5


Thin nitride

Thick nitride


Sealed - Interface Local Oxidation (SILO)

Key Ideas: Use mask-stack engineering to increase the stiffness of the oxidiation mask.

More resistance to bending during oxide expansion will give less lateral oxide growth.

Nitride, oxynitride, poly can all be used as the stiffeningg layer. It is important that the

stiffing layer be keep thin so as not to generate too much stress during oxidation.

Oxide Encroachment 0.2m/side

Can be fully recessed (etching Si

substrate ) or semi-recessed.

Mask Stack Engineering


(1) no pad oxide

Si3N4

Si crystal defects

Silicon Nitride as oxidation mask

Si3N4

Si

Si3N4

Si

SiO2oxidation

Nitride can be

oxidized at high temp(2) nitride too thin


SideWAll Masked Isolation (SWAMI)

nitride1

nitride2Pad oxide

Si(100)

(1)

(2)

(3)

(4)

•Can grow thick oxide with little lateral encroachment

•High thermal budget still has dopant diffusion problem


Solutions:

Use oxidation temp > 960°C to relieve stress by viscous flow of SiO2. High pressure

oxidation also helps.

Use thinner nitride and rounded etch corners (stress reduction)

Use groove sidewall angle < 75 °. This enhances the oxide viscous flow.

Defects with recessed LOCOS


Oxidation of Metal Silicides

• SiO2 formed on top of silicide in most cases.

Exceptions : MOx forms on top for HfSi2 and TiSi2 if oxidized

below 900oC

•Silicide layer is conserved : Si supplied by poly-Si or silicide


Oxynitridation by Nitrous oxide N2O

Gas Phase reactions

Interface reactions

Si +O2 SiO2 reaction constant k1

Si +NO SiOxNy reaction constant k2

Film reaction

SiOxNy +NO SiO2 +N2 reaction constant k3


O2 and NO

SiO2

and SiOxNy

Lower N

Content in

grown film


Growth rate slows

down attributed

to the

incorporation of

nitrogen

at silicon/silicon

oxynitride

interface.


Nitrogen accumulation at

the Si–SiO2 interface

improves:

•Radiation hardness

•Barrier properties to

impurity penetration,

•Stress stability

•Smaller charge trapping

•Large charge to

breakdown. (QBD)

50% N20

40% N20

Singhvi and Takoudis

JAP, Vol82, No.1, 442 (1997)

Nathan Cheung 14

Oxidation Affects DiffusionOxidation Enhanced

Diffusion (OED):

Interstitials created by

excess Silicon from

oxidation diffuse rapidly

and act as additional

stepping stones for the

dopant atoms.

Oxidation Reduced

Diffusion (ORD): The

dopant uses vacancies

and the population of

intersitials reduces

the poplulation of

vacancies.EE243 Sp2011 Lec 5


Thermal Oxidation creates volume change

(For 1-D growth, 1 m SiO consumes 0.46 m Si).

There will be mechanical stress generation and oxide flow.

1-D Deal-Grove Model: parameters ( D and ks); process variables (

pressure, oxidant, temperature, time, dopant conc, orientation).

2-D Deal-Grove model : has to include curvature and mechanical stress

effects

Thin oxide and oxynitride growth: need extended models

Interface Charges can be minimized by H annealing

Issues with nitride as oxidation mask

Other Topics: Oxidation enhanced diffusion, dopant redistribution, oxidation

induced stacking fault will be discussed in Diffusion Module

Summary of key concepts in thermal oxidation

Documents

Polycrystalline Si Oxidation